1. Field of the Invention
The present invention relates to ring laser gyroscope including multioscillator ring laser gyroscopes, and more particularly, to a solid state multioscillator ring laser gyroscope exhibiting the split gain effect.
2. Description of Related Art
Over the past twenty years, the gaseous medium planar ring laser gyroscope has been developed and evolved as a reliable and relatively environmentally insensitive inertial rotation sensor. Planar ring laser gyroscopes, of both triangular and square geometries, have been used in inertial navigation systems and flight control systems regularly in both commercial and military aircraft. A primary advantage of the ring laser gyroscope over the spinning wheel mechanical gyroscope is its ability to withstand relatively large mechanical shock without permanent degradation of its performance. Because of this and other features the expected mean time between failures of most RLG inertial navigation systems are somewhat longer than the mechanical gyroscopes they replace. The planar ring laser gyroscope was a first attempt at a non-mechanical truly strap-down inertial navigation system.
At low rotation rates, the retroscatter from the mirrors couples energy from one of the oscillating beams into the oppositely propagating beam which locks the oscillating frequencies together yielding zero rotation information at low rotation rates. Current operational ring laser gyroscopes having a planar configuration use mechanical dithering schemes to bias the rate sensor to avoid this well known lock-in phenomenon. Mechanical dither is very effective in reducing the effects of lock-in and makes the ring laser gyroscope a viable navigational gyroscope. However, an effective mechanically dithered ring laser gyroscope adds a noise component to the output of the ring laser which in turn reduces its ultimate accuracy. Also, the presence of mechanical dither, either in the mirror or dither of the entire gyroscope body, detracts from the desired goal of a fully strapped down inertial navigational unit.
With these problems in mind, an alternative biasing techniques have been developed using the nonreciprocal Faraday effect in either an application of a magnetic field directly to the gain medium known as the Zeeman effect, or a solid glass element known as a Faraday glass, which when used in combination with the magnetic field, provides a Faraday effect phase shift for one beam that is opposite the phase shift of the oppositely directed beam whereby two counter rotating beams are split infrequency. To achieve actual phase shifts instead of simple polarization rotation, two pairs of oppositely directed circularly polarized beams are optimally present within a single optical path to achieve a desired result. An example of this theory of multioscillator ring laser gyroscope may be found in U.S. Pat. No. 4,818,087 entitled "ORTHOHEDRAL RING LASER GYRO" issued Apr. 4, 1989 to Raytheon Corporation (Terry A. Dorschner, inventor). The nonplanar ray path produced in a multioscillator ring laser gyroscope insures circularly polarized light with reciprocally split frequencies. The nonplanar ray path reciprocally rotates the polarizations by many degrees yielding the necessary high purity circular polarization. The nonplanar reciprocal phase shift also achieves two Faraday bias gyroscopes, the gain curve 10 of which is illustrated in FIG. 1. The nonplanar ray path splits the light through its geometry into two separate gyroscopes, one being left circularly polarized and the other right circularly polarized. This splitting is known as reciprocal splitting and typically is in the range of 100 MHz. By placing a Faraday element in the beam path of a nonplanar ring laser gyroscope, when the proper magnetic field is applied to the Faraday glass element, nonreciprocal splitting of each gyroscope is achieved. At least four modes are produced: a left circularly polarized anti-clockwise frequency (L.sub.a), a left circularly polarized clockwise beam (L.sub.c), a right circularly polarized clockwise beam (R.sub.c), and a right circularly polarized anti-clockwise beam (R.sub.a). The Faraday splitting between clockwise and anti-clockwise modes is about 1 MHz. At least four mirrors form the ring resonator path, which contains the two gyroscopes symbolized by their respective gain curves of FIG. 1. One of the mirrors is semitransparent to allow light to leave the resonator and fall upon a photo detector for signal processing. When the signals are subtracted during the electronic processing to remove the Faraday bias, the scale factor of the gyroscope is doubled over the conventional ring laser gyroscope. The nonplanar geometry multioscillator ring laser gyroscope using a Faraday element is currently manufactured using a gas discharge to provide the active medium, which medium occupies a portion of the light beam path.
FIG. 2 shows an alternative form of ring laser gyroscope, through the diagram of its gain curve 20 A and B, which is known as the split gain gyroscope. The operation of this gyroscope is more fully described the patent application entitled "Split Gain Multimode Ring Laser Gyroscope and Method" Ser. No. 07/115,018, Filed Oct. 28, 1987 (Graham Martin, inventor), and assigned to the same assignee of this application. Ser. No. 07/115,018 is currently under United States Patent Office Secrecy Order (Type One Order). A brief explanation of the split gain gyroscope may be understood by reference to the gain curve 20 A and B of FIG. 2. Rather than operate in a single longitudinal mode as the multioscillator ring laser gyroscope of FIG. 1, the split gain gyroscope curve 20 A and B of FIG. 2 results from a nonplanar optical path to achieve reciprocal splitting setting up two separate gyros. However, the four different frequencies of the split gain gyroscope operate along more than a single longitudinal mode. The proper application of a uniform magnetic field in a gain regional of a split gain gyroscope allows one to achieve the equivalent of a Faraday bias by suppressing two of the four modes in each set of the longitudinal frequencies. These axial magnetic fields depend on the free spectral range of the cavity and typically have an average value of about 400 gauss. The axial fields are not used to create a Faraday bias as in the Faraday multioscillator gyroscope, but instead are used to effectively suppress the lasing action of two of the four gyro modes in each longitudinal mode set. For example, a first set of longitudinal frequencies, below Curve 20A, (as shown in FIG. 2), namely, the left circularly polarized clockwise (L.sub.c) and the right circularly polarized anti-clockwise (R.sub.a ) components of a first longitudinal set (q mode) and reciprocally split by a frequency difference 21, are suppressed in that the threshold 22 of the gain curve 20A is above the lasing frequencies of these two modes. "q" is an integer denoting a longitudinal mode number which is an integer N.) As a result, for the q mode, only the left anti-clockwise circular mode (L.sub.a) and the right clockwise circular polarized frequency (R.sub.c) remain, lasing under Curve 20A. The opposite effect may be had in the q+1 mode (Curve 20B), so that the left circular polarized clockwise mode (L.sub.c) and the right circular polarized anti-clockwise mode (R.sub.a) remain, and are split non-reciprocally by a frequency difference 23. By operating over a frequency range of eight potential circular polarized frequencies (L.sub.a, R.sub.c, L.sub.c, and R.sub.a), and then suppressing four of these frequencies (L.sub.c, R.sub.a, L.sub.a, and R.sub.c), the effect of reciprocal splitting (through use of a nonplanar path) and the nonreciprocal splitting (through use of mode suppression rather than the Faraday effect) achieves an operational multioscillator ring laser gyroscope without the need for a Faraday element other than the gas medium. Additionally, the split gain gyroscope operates so that the two respective gyroscopes (Splittings 21 and 23) are separated by a Free Spectral Range of 1000 MHz or more, while each of the sets of lasing longitudinal modes (L.sub.a, R.sub.c and L.sub.c, R.sub.a) are reciprocally split by approximately 100MHz or more, rather than 1 MHz as in Faraday biased Multioscillator ring laser gyroscopes. Furthermore, the split gain multioscillator ring laser gyroscope is not only a monolithic device but also has such a large bias that backscatter effects become secondary.
All the above ring laser gyroscopes are strapped down alternatives to the dithered planar ring laser gyroscope or the mechanical gyroscope. Both the multioscillator and the split gain gyroscope currently use a gaseous active medium. Gas discharge pumping requires expensive vacuum processing. This gas processing expense is not greatly reduced as the gas laser size is reduced. Additionally, active medium gaseous ring lasers are subject to life time degradation due to sputtering at the cathode of a DC excited active ring laser gyroscope.
Recent work in the ring laser solid state field and particularly optically pumped Nd:YAG lasers appear to be promising as a potential substitute for gaseous laser sources. U.S. Pat. No. 4,578,793, entitled "SOLID-STATE NON-PLANAR INTERNALLY REFLECTING RING LASER", issued Mar. 25, 1986 to Stanford University (Kane and Byer, inventors) disclosed a solid state nonplanar internally reflecting ring laser, the operation of which was further described in an article in Lasers and Optronics entitled Diode-Pumping The Nd:YAG Laser, LASERS & OPTRONICS, (Jun., 1987) 57-59. This article indicated that one might consider a Neodymium-Yttrium aluminum garnet material (Nd-YAG) may be used to provide a totally internally reflected ring laser source. Rather than using a gas discharge to excite the active medium, the solid state Nd-YAG laser proposed in this article would use an optically pumped source, typically in the form of a laser diode. According to the article the potential for an efficient pumping process may be achieved using a laser diode with a wave length between 795 and 805 nanometers to pump the Nd-YAG glass. When such a material is end pumped, it is easy to achieve a TEM.sub.oo transverse operational mode, which is needed for proper operational ring laser gyroscope or ring laser source. A solid state nonplanar internally reflecting ring laser, substantially as described in the '793 patent has recently been made available for commercial use by Light Wave Electronics Corporation of Mountain View of California. The teachings of the '793 patent operate in such a manner that the entire prism shown in FIG. 3 has a number of reflecting surfaces to provide an out of plane path. The entire prism is placed in the magnetic field creating a higher lost in one direction than the other, thereby forming an oscillator which includes an optical diode. The inventor claims to achieve frequency stability allowing coherent detection of laser radiation with a low band width detector for communication purposes. (Also, consider the articles by Bingkun Zhou, Thomas J. Kane, George J. Dixon, and Robert L. Byer entitled "Efficient, frequency-stable laser-diode-pumped Nd:YAG laser" (OPTICS LETTERS), Vol. 10, No.2, pp. 62-64 (February, 1985) and Thomas J. Kane and Robert L. Byer entitled "Monolithic, unidirectional single mode Nd:YAG ring laser" (OPTICS LETTERS), Vol. 10, No.2, pp. 65-67 (February, 1985). Both articles are directed to solid-state optically pumped lasers.)
In U.S. Pat. No. 4,747,111 issued to Hewlett Packard Company May 24, 1988, entitled "QUASI-PLANAR MONOLITHIC UNIDIRECTIONAL RING LASER" (Trutna, Inventor), the monolithic unidirectional single mode Nd-YAG ring laser was a basis for the disclosure of the '111 patent. The unidirectional ring laser of the '111 patent provides a stable single mode 1.3 micrometer output when placed in a magnetic field of 100 Gauss. The design embodied in this patent is directed to a monolithic semiconductor laser suitable for optical fiber communications.
In U.S. Pat. No. 4,829,537, entitled "SOLID STATE LASERS WITH SPHERICAL RESONATORS" issued May 9, 1989 to SpectraPhysics Inc. (BAER, inventor), a spherical resonator is disclosed which produces laser radiation having spherical modes of oscillation entirely within this sphere of the laser gain. A toroid region within this sphere forms a resonator. The design is envisioned for use in conjunction with the prism for input and output to the couplings of fiber optics. Alternatively it is suggested that the toroidal resonator forms a ring laser that may be used as a gyroscope.
The resonator described in this patent is accomplished with an optically pumped source. In this manner, the prior art has suggested the use a solid state resonator for ring laser gyroscope application.